1. Field of the Invention
The present invention relates to a circuit for controlling a plurality of ultrasonic motors, and more particularly, to a control system for a plurality of downsized ultrasonic motors which is suitably mounted on an electronic appliance.
2. Description of the Related Art
A servo motor is generally used for a positioning control mechanism such as an electronic appliance, and a motor is so driven as to move a member to be positioned to a desired position. In this case, a servo system is structured such that a present position of the member to be positioned is detected, a difference between the present position and the desired position is found and fed back to a drive portion of the motor to thereby control the driving of the motor so that a deviation signal becomes 0.
Because the motor used in the positioning control mechanism is generally of an electromagnetic type, the motor is susceptible to a disturbance vibration in a state where the member to be positioned reaches the desired position and a drive current does not flow, and there frequently occurs a phenomenon that the member to be positioned is deviated from the desired position. Therefore, even in the case where the desired position is not changed and the member to be positioned is originally in a stop state, when a positional displacement occurs due to the disturbance, the servo system is so actuated to be driven as to return the member to be positioned to the desired position. That is, because a conventional positioning mechanism has a property which is susceptible to the disturbance, there arises such a defect that an excessive electric power that corrects the influence is consumed.
As a motor having no defect that the positioning displacement is liable to occur due to the disturbance, attention has been paid to a vibration motor which is represented by an ultrasonic motor. This is because the vibration motor vibrates a vibrator having an elasticity due to a piezoelectric element or the like and imparts a drive force to a member to be driven as a rotor (or a slider) which is brought in pressure contact with the vibrator, and therefore even if the vibration stops, the member to be driven is structurally in pressure contact with the vibrator, which is equal to a kind of brake actuating state, and has a characteristic (self-holding function) that the member to be driven is difficult to be influenced by the disturbance vibration. Then, an attempt that as the servo motor of the positioning control mechanism, the conventional electromagnetic motor is replaced by the ultrasonic motor has been disclosed in JP 2001-268955 A, “Vibration motor, positioning device and method of controlling the vibration motor”, or the like.
Recently, a demand that the positioning mechanism is intended to be incorporated into various electronic appliances increases, and it is an important subject that the device is not large-sized. From this viewpoint, the application of the ultrasonic motor that can obtain a required drive force by a downsized motor is first advantageous in the downsizing of the device as compared with the electromagnetic motor. However, in the case where the ultrasonic motor is applied, because, particularly in a device having a plurality of members to be driven, it is necessary not only to provide a plurality of ultrasonic motors for driving them but also to provide the same number of control circuit s for controlling the driving of the respective servo systems as that of the motors, there arises a problem in that the device is not prevented from being large-sized.
Also, because the member to be positioned may be deviated from the desired position due to the disturbance or the like even if the ultrasonic motor having a self-holding function is used, a conventional servo control is consequently required in order to obtain a system with high reliability. Then, because a plurality of motors conduct different driving, respectively, the control circuit becomes complicated, and a control IC (particularly gate/array) is large-sized.